Not Applicable
Not Applicable
The invention relates to a system for detecting the presence of moisture, comprising at least one electronic sensor for detecting the presence of moisture and at least one reading device for obtaining information from the at least one sensor about the presence of moisture, wherein the at least one sensor comprises a resonant circuit which is at least partly formed from a moisture sensitive material, the electrical resistance of which changes when the material comes into contact with moisture, the reading device comprises means for generating an electromagnetic interrogation field comprising at least one frequency component corresponding to a resonance frequency of the resonant circuit and for recording the response of the at least one sensor to the electromagnetic interrogation field to obtain information about the presence of moisture at the at least one sensor.
Such a system is known from DE 40 30 284. In the known system, the reading device is provided with a measuring circuit comprising a transformer connected to the resonance circuit. The resonant circuit comprises a material from which the electrical resistance decreases when the material comes into contact with moisture.
It is a drawback of the known system that the information generated by the sensor about the presence of moisture is often not sufficiently reliable. Moreover, such a system is rather expensive and thus less suitable for use as a disposable sensor, because the reading device and the sensor are part of one and the same device. Furthermore the system is not suitable for wirelessly obtaining information about the presence of moisture.
The invention has for its object, inter aila, to meet the above drawbacks and, furthermore, to provide a number of advantages.
The system according to the invention is accordingly characterized in that the electrical resistance of the material (18) increases when the material comes in to contact with moisture and the reading device (4.1) comprising transmitter-receiver means (14) for wirelessly generating the electromagnetic interrogation field and for wirelessly recording the response of the at least one sensor (2.i) to the electromagnetic interrogation field to obtain the information about the presence of moisture at the at least one sensor (2.i). if the at least one sensor (2.i) is wirelessly brought into the electromagnetic interrogation field.
It has been found that the effect of moisture on the moisture sensitive material and thus the presence of moisture at the sensor can be recorded very sensitively and accurately. When the moisture sensitive material comes into contact with moisture, the electrical resistance will increase. Because of the increase in the electrical resistance, the electrical properties of the resonant circuit will change and the response of the resonant circuit to the interrogation field will thereby also change. In this connection it is even conceivable that in this manner not only the presence of moisture at the sensor is detected, but that even an impression can be obtained of the amount of moisture present at the sensor.
The sensor according to the invention can be used, inter alia, in baby diapers, incontinence diapers, sanitary towels, incubators, packages for vegetables and fruit, on the road surface for detection of rain and at a substratum in the cultivation under glass. It is also possible to use the sensor in drying processes, such as, for instance, in the paper industry.
GB 21 92 059 discloses a system comprising a wetness sensor for wirelessly reading the wetness sensor. The system is for measuring the moisture content of oil in a container. The wetness sensor comprises a moisture detector, an active transmitter and an antenna for generating a modulation of the transmission signal according to the resistance of the moisture detector by changing the q-factor of the antenna. In this known system energy is supplied to the sensor by means of mechanical vibration submitted to an outerwall of the container comprising the sensor.
EP-A-0 329 436 discloses a moisture and dew detection sensor comprising a fabric and a moisture sensing resistive substance adhered in a substantially continued and dispersed state to the fabric.
The resistance of the fabric increases if it comes into contact with moisture. Furthermore, the resistance of the fabric is measured by a non-wireless connection to a measuring unit. The system is, however, not provided with a reading unit which generates an interrogation field with a frequency which corresponds with a resonance frequency of a resonance circuit of the sensor so that the resonance circuit is brought to resonance by means of the interrogation field.
Preferably, it applies that the moisture sensitive material is included in the resonant circuit in such a manner that the Q factor of the resonant circuit decreases when the resistance of the moisture sensitive material increases. The Q factor of the intact dry sensor is therefore high. This means that the sensor can be properly detected in this condition. The system can therefore also be used to check whether a sensor is present in the product (such as, for instance, a diaper). This possibility is not present at the above prior art sensor, because this sensor does not react when no short circuit is present between the two electrodes.
A further advantage is that the change in the characteristic of the sensor is reversible. When the sensor dries again, the resistance of the moisture sensitive material will decrease.
In the above special embodiment this means that the Q factor of the resonant circuit increases again.
According to a special embodiment it applies that the resonant circuit at least comprises an LC circuit. In this connection the entire LC circuit or at least part of the LC circuit may be built up from the moisture sensitive material
In particular, it applies that the moisture sensitive material comprises a binding agent capable of swelling in moisture, in which binding agent electrically conductive particles are included. It is also possible that the moisture sensitive material comprises a binding agent in which particles capable of swelling in moisture and electrically conductive particles are included. In both cases moisture ensures a swelling of respectively the binding agent and the particles capable of swelling. Consequently, the electrically conductive particles will be drawn apart and the conductivity of the moisture sensitive material will decrease so that the electrical resistance of the material increases.
In particular, it applies that the reading device generates an alarm signal when moisture is detected by means of the sensor.
According to a very advanced embodiment of the invention the system is also designed as an identification system in which the at least one sensor comprises an active electronic circuit connected with the resonant circuit, such as a microprocessor in which an identification code is stored, which identification code is passed to the resonant circuit when the resonant circuit is resonated by the electromagnetic interrogation field, and the reading device being arranged to read the identification code by means of the electromagnetic interrogation field.
This system can, for instance, advantageously be used in a hospital, the sensor being used to record moisture in a mattress of a hospital bed. Each sensor may then comprise an identification code belonging to a specific hospital bed. In this manner it is not only possible to record that a mattress has become wet, but also which mattress has become wet.
The system may further comprise a central control unit which is, optionally wirelessly, connected with the at least one reading device for obtaining information about the presence of moisture at the at least one sensor.
In the example of the above hospital the central control unit can be installed, for instance, in the room of a nurse. The reading devices can be installed in the different rooms of the patients. In this manner it can be centrally recorded in which room which bed has got a wet mattress.